The purpose of this project is to evaluate the risk of thyroid nodularity among patients exposed to low-dose radioactive iodine (I-131). Physical examinations of the head and neck will be performed on two Swedish populations: 1000 women with a history of exposure to diagnostic doses of I-131, and 250 general population comparisons undergoing mammographic screening. Risk will be evaluated over a range of doses of diagnostic I- 131, and risk for the low dose women will be compared with that for the general population comparison group. The exposed population will be selected from previously identified patient cohorts and the mammography screenees will be chosen from women participating in national screening programs. The rates of detected thyroid diseases known to be related to low-dose radiation (cancer, adenomas, nodules) and those not so related (e.g., hyperthyroidism, thyroiditis, hypothyroidism) will be compared in the study groups and by dose, and other biological markers of exposure (e.g., fluorescent in situ hybridization translocation analysis) will be evaluated to learn more about biological markers as potential dosimeters.